A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy
(ān'yə-ploi'dē) (an abnormal number of chromosomes).

Description and Causes:
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes. If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes. If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

Terminology:

"Full trisomy", also called "primary trisomy", means that an entire extra chromosome has been copied. *"Partial trisomy" means that there is an extra copy of part of a chromosome.

"Secondary trisomy" - the extra chromosome has duplicated arms (the arms are identical; it is an "isochromosome").

"Tertiary trisomy" - the extra chromosome is made up of copies of arms from two other chromosomes.

Trisomies are sometimes characterised as "autosomal trisomies" (trisomies of the non-sex chromosomes) and "sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy. Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.

Human Trisomy
Trisomies can occur with any chromosome, but often result in miscarriage, rather than live birth. For example, Trisomy 16 is the most common trisomy in human pregnancies, occurring in more than 1% of pregnancies; only those pregnancies in which some normal cells occur in addition to the trisomic cells, or mosaic trisomy 16, survive. This condition, however, usually results in spontaneous miscarriage in the first trimester.

The most common types of autosomal trisomy that survive to birth in humans are:

Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, intellectual disability and shortened life expectancy.

Mosaic Down syndrome is usually detected by a blood test at birth or during an amniocentesis or CVS during pregnancy. A karyotype analysis is done, and typically the chromosomes in 20 different cells are counted. If two or more of the 20 are normal (have 46 chromosomes) and the others have an extra number 21 chromosome (for a total of 47 chromosomes), the baby is said to have mosaic Down syndrome.

If mosaicism is suspected, the cytogeneticist (chromosome specialist) will usually count extra cells to confirm the diagnosis. Mosaicism is usually described as a percentage. For example, if the cytogeneticist counts 50 cells, and 10 have a normal cell line and 40 have an extra number 21 chromosome, then the level of mosaicism is said to be 80%.

Percentage Mosaicism:

While the percentage of mosaicism is usually determined from blood cells, the percentage may actually be different in other tissues or parts of the body. For example, a person could have 80% mosaicism in their blood cells, but have full trisomy in their skin or brain cells. While it is known that the percentage of mosaicism can be different in different tissues, biopsies are not usually done because they are invasive and usually don't give any information about how a person with mosaic Down syndrome will ultimately function.

Level of Mosaicism and Features of Down Syndrome:

Individuals with mosaic Down syndrome may or may not have milder disabilities and less obvious features of Down syndrome. Because levels of mosaicism vary between individuals and within the cells of the individuals themselves, the effect of mosaicism is wide and variable.

Individuals with mosaic Down syndrome can have all of the problems associated with the full trisomy 21, none of the problems, or somewhere in between.

Should I Treat My Baby with Mosaic Down Syndrome Different Than a Child with Trisomy 21?

No, every baby with mosaic or full trisomy 21 should be treated the same. Because it is impossible to predict function or features from the level of mosaicism, it is important that a child with mosaic Down syndrome receive the same medical care and therapies as a child with full trisomy 21. Only time will tell if the disabilities of a child with mosaic Down syndrome are milder than a child with trisomy 21.

In the meantime, it is important to make sure your child receives all the support she or he needs to live a full life and achieve their goals.